ML20212D368

Submits Assessment of Impact of Replacing Graphite Shims W/ Carbon Steel Shims on Hot Leg Pipe Whip Restraints.Potential Effect on RCS Piping in Event of Future Contact Between Hot Leg Piping & Whip Restraints Addressed

ML20212D368
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	12/19/1986
From:	Lindblad W PORTLAND GENERAL ELECTRIC CO.
To:	Varga S Office of Nuclear Reactor Regulation
References
TAC-61405, NUDOCS 8612310345
Download: ML20212D368 (3)

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PORTLAND GENERAL ELEcTHic COMPANY 121 S.W. S ALM O N STREET WILLIAM J. LIND8 lao Po RT LA N o. O R EGO N 97204 I "5" csos)aze-sa7s December 19, 1986 l

Trojan Nuclear Plant Docket 50-344 License NPF-1 Director of Nuclear Reactor Regulation -

ATTN: Mr. Steven A. Varga Director, PWR-A

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Project Directorate No. 3 U.S. Nuclear Regulatory Comission Washington DC 20555

Dear Mr. Varga:

Use of Carbon Steel vs Graphite Shims in Reactor Coolant System Your safety evaluation of June 16, 1986 for the inoperable steam generator snubbers on the Trojan Nucleer Plant reactor coolant loop, directed PCE to provide an assessment of the impact of replacing graphite shims with carbon steel shims on the hot les pipe whip restraints. This assessment is pro-vided herein and addresses the potential effect on the Reactor Coolant System (RCS) piping in the event of future contact between the hot leg piping and the whip restraints.

Since 1982, when a pressurizer surge line thermal sleeve was removed, unexpected movements of the pressurizer surge line had been observed. As a result of steam generator hydraulic snubber testing in 1985, all 16 snubbers were declared inoperable. One of the hypotheses from the evaluation of the pressurizer surge line movements was that the snubbers might have failed in such a manner so as to restrain normal RCS thermal expansion, thereby causing the observed movement. Subsequently, a thorough examination of RCS components was performed during the 1986 refueling outage. The examination suggested that the RCS piping had experienced abnormal movement in that damage to the graphite shims on each of the hot leg whip restraints was observed. The examination also revealed the gaps between the steam genera-tors and the upper and lower seismic support rings were insufficient to accomodate the expected thermal expansion of the RCS loops.

Following extensive snubber control valve testing and further inspection and evaluation of the RCS loop, it was determined that the inadequate gaps discussed above were the most likely cause of the damage to'the hot leg whip restraint shims. A detailed analysis of RCS thermal expansion was conducted and the whip restraint gaps were adjusted to the proper clearances. As part of this adjustment, the hot leg whip restraint graphite shims were replaced with carbon steel shims. The modified clearances were monitored during subsequent heatups and cooldowns and it was verified the RCS did not contact the whip restraints.

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e PORTLAND GENE!*AL ELECTRIC COMPANY Mh. Steven A. Varga i December 19, 1986 Page 2 Since the cause of the original RCS contact with the hot leg whip restraints was determined to be inadequate clearances on the steam generator support rings and possibly at the hot leg whip restraints themselves, and the cause has been corrected, it is not expected that the RCS will again contact the whip restraints. A temporary monitoring program has been developed to demonstrate the thermal expansion of the RCS occurs within predictable limits. This monitoring program will continue until this predictability has been established.

The pipe whip restraints are designed to limit movement of the pipe in the event of a pipe rupture. During normal operations, it is not intended for the pipe to contact the restraint. In the event the pipe was to contact the carbon steel shims during normal operations, finite element analyses have demonstrated under worst-case conditions the faulted load stress limits of the pipe would not be exceeded.

s A parceived benefit of the graphite shims was that they " crushed" under load, thereby relieving the stress on the RCS loop. Since it is not intended for the pipo to contact the restraint and our efforts have been to preclude such contact, this apparent advantaga has become a moot issue. Under pipe rupture conditions, it is not necessarily desirable for the shims to " crush".

In conclusion, it has been determined that the replacement of the graphite shims in the RCS hot leg whip restraints with carbon steel shims will not have an adverse effect. The RCS whip restraint gaps have been adjucted in order to prevent any further contact between the shims and the RCS piping and a tamporary monitoring program is being conducted to confirm contact does not occur. In the unusual event that contact should occur, the stresses impesed on the RCS would be acceptable.

l l Sincerely, eg sMe l

c: Mr. John E. Martin Regional Administrator, Region V U.S. Nuclear Regulatory Commission Mr. David Kish, Acting Director State of Oregon Department of Energy

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